Cellular materials can be made to combine high strength to weight ratio, elastic resilience and energy absorbing properties. They are attractive for lightweight structures, packaging and insulating purposes. Nature provides an abundance of examples in wood, leaves, cork, bone and many other organically built structures that display a wealth of intricate and ingenious shapes, forms and designs. Bone and its growth response to stresses is an example of the versatility and of the adaptability of cell structures to mechanical requirements.
Manmade cellular materials have found many applications because it is possible to tailor the properties for a variety of purposes, such as for packaging, insulating, or structural purposes. Open cell structures such as foamed polymers constitute perhaps the most widely used manmade cell materials. Significant developments have also been made in the fabrication of cell materials from metals, glass, ceramics and graphite. Honeycomb structures may be considered as cell structures with designed architecture. They find use in structural applications such as for lightweight aircraft components.
A variety of methods exist for producing cell structures of less well defined design. Such are the foaming methods. Metallic foams are well suited for a variety of applications including impact energy absorbers, silencers, filters, heaters, heat exchangers and structural parts, but more cost effective competitive materials are generally used. Open metal foams were investigated by Ford Motor Company in the early 1970""s, but they have not yet found significant use in motor vehicles. One reason for this may be that such foams can be easily crushed in compression due to buckling and plastic collapse of relatively thin cell walls.
The inventors presented their analysis of closed cell metal composites in their published article, M. Ozgur, R. L. Mullen and G. Welsch, xe2x80x9cAnalysis of Closed Cell Metal Compositesxe2x80x9d Acta Mater., Vol. 44, No. 5, pp. 2115-2126 ((copyright)1996) . They also presented the results of finite modelling of pressurized closed all composites in M. Ozgur, R. L. Mullen, G. Welsch, xe2x80x9cFinite Element Modelling of Internally Pressurized Closed Cell Compositesxe2x80x9d International Journal for Numerical Methods in Engineering, Vol. 39, pp. 3715-3730 ((copyright)1996).
Closed cell metal composites offer advantages over open cell metal foams. An object of this invention is to provide an artificial closed cell metal composite having desirable strength as well as damping or elastic properties. Depending upon the materials used in forming the closed cell metal composites, the resulting bodies can be compatibly adapted for use in a variety of applications ranging from biomedical prostheses to automotive brake disks, various castings and to structural parts.
A closed cell metal composite material comprises a plurality of closed structural metal cells that are joined together into an aggregate arrangement. Each cell encapsulates a fluid or fluid-like filler therein in order to provide strength and damping characteristics to the material. Fluid or fluid-like materials may be gases, liquids, powders and solids of relatively low elastic or plastic deformation strength in relation to the cell wall material. The aggregate arrangements of cells serve to provide crush resistance in compression. The cells are at least one nanometer in size. This technology may be utilized in a variety of situations and the selection of the appropriate cell and filler materials coincides with the intended application. Also, it is important to select mutually compatible cell and filler materials.
For example, in the case of biomedical prosthesis, the outer material of the cell walls will necessarily be comprised of a biocompatible material, quite likely titanium or a titanium alloy or gold alloy, stainless steel, cobalt alloy or any metal deemed to be biocompatible. The biomedical cells may encapsulate pressurized gas, or a polymeric material, or a relatively soft metal or alloy of such elements as Li, Na, K, Rb, Cs, Mg, Ca, or a powdered material such as graphite, or a powder or paste or slurry of a substance that is relatively nonreactive and which may have the additional attribute of being biophile. The attribute xe2x80x9crelatively softxe2x80x9d is in relation to the elastic and plastic rigidity of the cell wall material. The attribute xe2x80x9crelatively nonreactivexe2x80x9d is firstly in relation to the metal of the cell wall during processing at low and elevated temperature and during the use lifetime of the cell composite, and it is secondly in relation to the biological environment if the cell composite is used for prosthesis and if the cells"" filler substance, either intentionally or unintentionally, can come in contact with the biological environment. The attribute xe2x80x9cbiophilexe2x80x9d relates similarly to biological environment if the filler substance can come into contact with the biological environment. Filler substance can be one or several of the following: stable borides, carbides, nitrides, oxides, and sulfides, also stable fluoride, chloride, bromide and iodide salts, also stable oxynitrides, carbonates, phosphates, sulfates. Examples of boride powder, paste or slurry substances for cell filling are borax and boron nitride; examples of carbide powder, paste or slurry substances for cell filling are silicon carbide and titanium carbide; examples of nitride powder substances for cell filling are silicon nitride and cobalt nitride; examples of oxide powder substances for cell filling are H2O, aluminum oxide, silicon dioxide and titanium oxide, calcium oxide and magnesium oxide; an example of sulfide cell filling is molybdenum disulfide; examples of fluoride, chloride, bromide and iodide salts for cell filling are lithium fluoride, calcium fluoride, sodium chloride, potassium chloride, magnesium bromide, and potassium iodide. The salt fillers may be powders, or liquids, or slurries or relatively soft solids.
Powdered materials such as graphite, calcium oxide, calciumcarbonate, hydroxyapatite bioglass may be encapsulated. When the filler is graphite, carbon will form a titanium carbide layer of a few micrometers thickness along the interior surface wall of the titanium cell wall during elevated temperature processing. This thin film will stop any further reaction of the graphite with the titanium. Calcium and calcium oxide compounds resist being dissolved into the titanium and are therefore stable filler materials during elevated temperature processing. The hydroxyapatite powder is desirable because it offers biocompatibility, as apatite is a main component in human bone. Biocompatibility of the cell filler materials is desired for the event that some of the cell walls in a prosthesis are opened either intentionally or accidentally to the biological environment. The biomedical materials will preferably call for cell structures in which individual cells have sizes ranging from 100 micrometers to roughly a centimeter. The pre-formed closed cells are likely hot-pressed or fusion-bonded together to form the aggregate material from which any number of biomedical items may be formed including hip or joint replacements, rigid fixation devices, pins, nails and dental implants or other prosthetic devices.
When the closed cell materials are intended for use in automotive components such as brake disks, the cells may be comprised of shells made of structural metals that have melting temperatures higher than 500xc2x0 C., such as steel, and which encapsulate powdered graphite or MoS2. Graphite and MoS2 provide desirable tribological properties to friction brake systems. These cells are joined together in an aggregate body and provide a desirable material for use as a brake disk. Any remaining spaces between the cells of the aggregate may either be left empty or may be filled by liquid infiltration with Al, Mg, Si, casting iron, or a lower melting material than the cells walls.
The closed metal cell composite materials of the present invention are formed by encapsulating a fluid or fluid like filler with a structural metal to provide a filled structural metal closed cell. A plurality of filled structural metal closed cells are then arranged or formed into an aggregate body. The cells are bonded together by sintering, form-pressing, or fusion bonding. A binder may be added to fill the interstices between the cells, if any, and to join the cells together if necessary.
An advantage of the present invention is that the resulting composite material is more lightweight and less dense than the traditional materials they replace. The new composite material provides a way to position a composite blend in an environment without the added weight of traditional materials. For example, disk brakes are traditionally comprised of cast iron, which contains some graphite. Both the iron and the graphite are needed in the operative functioning of the brakes. However, the prior art cast iron is much heavier than the graphite-filled steel closed cells of the present invention.
Another advantage is that when the closed cells comprise titanium and the filler is graphite, these may be aggregated together to form prosthetic devices. The elastic moduli and compliances of the resulting composite are similar to those of bone.
Still other advantages of the invention will become apparent upon a reading of the detailed description herein.